1. Field of the Invention
The present invention relates to a display device. More particularly, the present invention relates to a liquid crystal display (LCD).
2. Description of Related Art
Currently, the performance of thin film transistor liquid crystal displays (TFT-LCD) are developed towards high contrast ratio, no gray level inversion, little color shift, high luminance, full-color, high color saturation, quick response, and wide viewing angle. At present, displays meeting the requirement for wide viewing angle include a twisted nematic (TN) LCD with a wide viewing film, an in-plane switching (IPS) LCD, a fringe field switching (FFS) LCD, a multi-domain vertical alignment (MVA) TFT-LCD, and so on. Here, an MVA-LCD panel is taken as an example. Alignment protrusions or slits formed on a color filter substrate or a TFT array substrate make liquid crystal molecules be arranged in different directions, so as to obtain multiple domains, such that the MVA-LCD panel can meet the requirement of wide viewing angle.
FIG. 1A is a graph illustrating a relation between normalized transmittance and gray level of a conventional MVA-LCD panel. Referring to FIG. 1A, the horizontal coordinate is the gray level, and the vertical coordinate is the normalized transmittance. As shown in FIG. 1A, though the conventional MVA-LCD panel meets the requirement of a wide viewing angle, the transmittance-level curve has different curvatures with the change of the observation viewing angle. In other words, when the observation angle changes, the brightness of the conventional MVA-LCD varies accordingly, leading to problems such as color shift or color washout.
Many conventional techniques have been proposed to solve the problem of color shift or color washout. Among them, a method is proposed by forming two pixel electrodes in a single pixel structure, and using a capacitive coupling effect to make the voltage of one pixel electrode be at a fixed ratio of that of the other pixel electrode, such that different pixel electrodes in a single pixel structure respectively generate different electric fields, so as to make liquid crystal molecules above the pixel electrodes be arranged in different manners. Though the above method can solve the problem of color shift or color washout, the pixel voltage generated by capacitive coupling may easily shift, which further results in display distortion.
FIG. 1B is an equivalent circuit diagram of the pixel structure of the conventional MVA-LCD panel for solving the above problem of voltage shift. Referring to FIG. 1B, the pixel structure 10 is electrically connected to a scan line 11 and a data line 12. The pixel structure 10 includes a first active device 20 electrically connected to the scan line 11 and the data line 12, a first pixel electrode 30 electrically connected to the first active device 20, a second pixel electrode 40, and a second active device 50 electrically connected to the second pixel electrode 40. The second pixel electrode 40 is coupled to the first pixel electrode 30 through a coupling capacitor 60. The second pixel electrode 40 generates a second pixel voltage Vp2 through the coupling capacitor 60 under the coupling effect of a first pixel voltage Vp1 of the first pixel electrode 30. As shown in FIG. 1B, the second pixel voltage Vp2 of the second pixel electrode 40 is stabilized by the second active device 50.
Referring to FIG. 1B, though the second active device 50 is capable of eliminating the problem of voltage shift, as the second pixel voltage Vp2 is influenced by a kick-back voltage generating from the second active device 50, the average second pixel voltage Vp2 of the second pixel electrode 40 is quite different from the average first pixel voltage Vp1 of the first pixel electrode 30, thus resulting in flicker of the LCD panel. Further, when the first active device 20 is turned-on to perform a charging procedure on the first pixel electrode 30, the second active device 50 is turned-on at the same time to perform a charging procedure on the second pixel electrode 40. As a result, the second pixel voltage Vp2 of the second pixel electrode 40 may shift, resulting in the change of the original display gray level. Additionally, since the device characteristic (for example, width/length ratio of the channel layer, W/L) of the second active device 50 is closely related to the display gray level of the second pixel electrode 40, the display stability of the LCD will be adversely affected when the line width or alignment of the second active device 50 varies due to process errors.